The coatings industry continues to develop new chemistries as performance requirements for decorative and functional coatings evolve. Drivers for change are varied and these can include: regulatory controls to reduce VOC emissions, concerns about toxic hazards of coating raw materials, a desire for cost reduction, commitments to sustainability, and a need for increased product effectiveness.
UV nail gel coatings have gained rapid popularity with fashion conscious individuals who apply nail polish to fingernails or toenails to decorate and protect nail plates. UV nail gels can produce coatings that exhibit phenomenal chip resistance and durability when properly applied and cured in comparison to those nail coatings derived from traditional solvent based nail lacquers. The performance difference particularly becomes apparent when the coating is applied on human finger nails and tested for durability. UV nail gel coatings can easily last for two weeks or more and still look like new whereas conventional nail polishes are easily scratched and will chip or peel from the natural nail in one to five days. UV nail gels are typically based on acrylates that cure quickly into dense, crosslinked thermoset coatings within half a minute or so. This is an advantage as the coating becomes almost immediately resistant to denting and scratching. Conventional nail lacquers show significant sensitivity to denting while the solvent evaporates from the coating and this requires great care by the individual as the coating dries and hardens; a process that can take easily fifteen to twenty minutes. However, conventional nail polish is easily removed with solvent whereas it can take some effort to remove a fully cured UV nail gel from the nail surface. An expensive UV light also is required for UV nail gel application and this has limited the success of UV nail gels in the mass market for home use. The expense of a UV light is less of an issue for professional salons where a right balance between service rate and a customers' perception of service is more important. As such, there is a need in the consumer market place for durable nail coatings that can cure quickly but do not require procurement of an UV light.
Highly crosslinked, durable coating compositions can be achieved using Michael addition chemistry. The Michael addition reaction involves the nucleophilic addition of a Michael donor, such as a carbanion or another nucleophile to a Michael acceptor, such as an α,β-unsaturated carbonyl. As such, the base catalyzed addition of activated methylene moieties to electron deficient C═C double bonds are known in coatings applications. Representative examples of suitable materials that can provide activated methylene or methine groups are generally disclosed in U.S. Pat. No. 4,871,822, which resins contain a methylene and/or monosubstituted methylene group in the alpha-position to two activating groups such as, for example, carbonyl, cyano, sulfoxide and/or nitro groups. Preferred are resins containing a methylene group in the alpha-position to two carbonyl groups, such as malonate and/or acetoacetate group-containing materials, malonates being most preferred. The α,β-unsaturated carbonyl typically is an acrylate material and representative materials have been disclosed in U.S. Pat. No. 4,602,061. The Michael reaction is fast, can be carried out at ambient temperatures and gives a chemically stable crosslinking bond without forming any reaction by-product.
A typical crosslinkable coating composition comprises a resin ingredient A (Michael donor), a resin ingredient B (Michael acceptor) and a base to start and catalyze the Michael addition reaction. The base catalyst should be strong enough to abstract, i.e. activate a proton from resin ingredient A to form the Michael donor carbanion species. Since the Michael addition cure chemistry can be very fast, the coating formulator is challenged to control the speed of the reaction to achieve an acceptable balance of pot life, open time, tack free time and cure time. Pot life is defined as the amount of time during which the viscosity of a mixed reactive system doubles. Working life or working time informs the user how much time they have to work with a reactive two part system before it reaches such a high state of viscosity, or other condition, that it cannot be properly worked with to produce an acceptable application result. Gel time is the amount of time it takes for a mixed, reactive resin system to gel or become so highly viscous that it has lost fluidity. The open time of a coating is a practical measure of how much time it takes for a drying or curing coating to reach a stage where it can no longer be touched by brush or roller when applying additional coating material without leaving an indication that the drying or curing coating and newly applied coating did not quite flow together. These indications normally take the form of brush or roller marks and sometimes a noticeable difference in sheen levels. The tack free time is the amount of time it takes for a curing or drying coating to be no longer sticky to the touch, i.e. the time for a system to become hard to the touch, with no tackiness. Cure time is the amount of time it takes for a coating system to reach full final properties.
The Michael reaction starts the very moment when coating resin ingredients A and B are mixed together with a suitable base. Since it is a fast reaction, the material in a mixing pot starts to crosslink and the fluid viscosity starts to rise. This limits the pot life, working time and general use as a coating. A dormant initiator that is essentially passive while coating material remains in a mixing vessel but that activates the Michael addition reaction upon film formation allows for longer pot life and working time, yet would show good open time, tack free time and cure time. Hence, the application of dormant initiator technology can provide the formulator with tools to control the speed of the reaction in order to achieve desirable cure characteristics.
U.S. Pat. No. 8,962,725 describes a blocked base catalyst for Michael addition, which is based on substituted carbonate salts. Preferred Michael donor resins are based on malonate and Michael acceptor resins are acrylates. The substituted carbonates can bear substituents, but these should not substantially interfere with the crosslinking reaction between malonate and acrylate. The carbonate salts release carbon dioxide and a strong base upon activation by means of film formation. The base is either hydroxide or alkoxide. Before practical pot life and gel times are achieved with acceptable curing characteristics, the carbonate requires presence of a certain amount of water in the coating formulation for the blocking of the base to become effective. All disclosed blocked carbonate examples utilize methanol and/or water. However, malonate esters are known to be susceptible to base hydrolysis, particularly when water is present. Hence, the water necessary to block the carbonate base can thus degrade malonate oligomers or polymers at the same time, which in turn can lead to altered coatings performance. The hydrolysis product furthermore can result in undesirable destruction of base catalyst by means of formation of malonate salt; a reaction which is cloaked as longer pot life and gel time. Presence of water can also be quite problematic in certain coatings applications. Wood grain raising is a significant problem when water is present in wood coatings; water penetrates into wood, which causes swelling and lifting of fibers and this leaves a rough surface. Water also can cause flash rust, i.e. appearance of rust spots on a metal surface during drying of newly applied paint that contains water. Longer term rust formation in terms of corrosion may also be a problem when dealing with formulations that contain water.